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Pick the statement which correctly describes the acceleration

  1. Sep 26, 2008 #1
    Pick the statement which correctly describes the acceleration, a, of the objects described below (neglecting air resistance). Select from A: a=0 m/s2, B: a=9.8 m/s2 down, C: a=9.8 m/s2 up, D: a > 9.8 m/s2 down, E: a < 9.8 m/s2 down, F: a > 9.8 m/s2 up, G: a < 9.8 m/s2 up, H: Not enough information E.g., if the first answer is A and the rest C, enter ACCCCC.
    1) An elevator moving upward at constant speed
    2) Just after a bullet is shot downward from a gun
    3) An elevator starting on its descent
    4) Just after release of an object which is thrown upward
    5) At the top of the path of an object thrown upward
    6) While a ball is in a person's hand during a downward throw




    I am very new to this, so sorry if I am too vague. Any help would be GREATLY appreciated.
     
  2. jcsd
  3. Sep 26, 2008 #2

    Redbelly98

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    Welcome to PF.

    Our policy is that you need to show an attempt at solving the problem before getting help.
     
  4. Sep 26, 2008 #3
    LOLZZZZZ OOPZZZ. Okay well, I have a feeling I may be overthinking this way too much. I thought it was ADBFBD, but I was told this is incorrect. I am unsure of 3) and 4), and this could be where I am wrong
     
  5. Sep 26, 2008 #4

    PhanthomJay

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    If (3) a=g, the el would be in free fall. Wouldn't want to be on that lift. In (4), the problem is looking for the acceleration after it leaves the thrower's hands.
     
  6. Sep 26, 2008 #5
    Okay so what exactly are you suggesting? I have 2 tries left and I really need to get this right. Perhaps one of the other four is wrong
     
  7. Sep 26, 2008 #6

    Redbelly98

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    I agree with #'s 1, 5, & 6. So think again about 2, 3, 4.

    Two things to think about for each case:

    1. What are the forces acting on the object? We know that gravity causes an acceleration of 9.8 m/s^2; are there additional forces present that make the acceleration different?

    2. Is the velocity changing for the situation described? If so, how (faster/slower, up/down)?
     
  8. Sep 26, 2008 #7
    Thank you so much for your help. The answer was ABEBBD. Those two pieces of advice you gave in the last post were a great help. Thanks again, sorry for any inconveniences with my posts.
     
  9. Sep 26, 2008 #8

    Redbelly98

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    You're welcome! No inconvenience, that's what this forum is all about.
     
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